expand: Add new clrsb fallback expansion [PR101950]

As suggested in the PR, the following patch adds two new clrsb
expansion possibilities if target doesn't have clrsb_optab for the
requested nor wider modes, but does have clz_optab for the requested
mode.
One expansion is
clrsb (op0)
expands as
clz (op0 ^ (((stype)op0) >> (prec-1))) - 1
which is usable if CLZ_DEFINED_VALUE_AT_ZERO is 2 with value
of prec, because the clz argument can be 0 and clrsb should give
prec-1 in that case.
The other expansion is
clz (((op0 << 1) ^ (((stype)op0) >> (prec-1))) | 1)
where the clz argument is never 0, but it is one operation longer.
E.g. on x86_64-linux with -O2 -mno-lzcnt, this results for
int foo (int x) { return __builtin_clrsb (x); }
in
-       subq    $8, %rsp
-       movslq  %edi, %rdi
-       call    __clrsbdi2
-       addq    $8, %rsp
-       subl    $32, %eax
+       leal    (%rdi,%rdi), %eax
+       sarl    $31, %edi
+       xorl    %edi, %eax
+       orl     $1, %eax
+       bsrl    %eax, %eax
+       xorl    $31, %eax
and with -O2 -mlzcnt:
+       movl    %edi, %eax
+       sarl    $31, %eax
+       xorl    %edi, %eax
+       lzcntl  %eax, %eax
+       subl    $1, %eax
On armv7hl-linux-gnueabi with -O2:
-       push    {r4, lr}
-       bl      __clrsbsi2
-       pop     {r4, pc}
+       @ link register save eliminated.
+       eor     r0, r0, r0, asr #31
+       clz     r0, r0
+       sub     r0, r0, #1
+       bx      lr
As it (at least usually) will make code larger, it is
disabled for -Os or cold instructions.

2021-08-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/101950
	* optabs.c (expand_clrsb_using_clz): New function.
	(expand_unop): Use it as another clrsb expansion fallback.

	* gcc.target/i386/pr101950-1.c: New test.
	* gcc.target/i386/pr101950-2.c: New test.

0 files changed, 0 insertions, 0 deletions